An infrared radiation having a wavelength of not less than 2 .mu.m is used in medical treatment, industrial machining, measurement, analysis, chemistry, and other various fields. In particular, an Er-YAG laser with a wavelength of 2.94 .mu.m, a CO laser with a wavelength of 5 .mu.m, and a CO.sub.2 laser with a wavelength of 10.6 .mu.m have high oscillation efficiency to provide high output and, at the same time, have large absorption for water, rendering these lasers important as light sources for medical/surgical treatment equipment and industrial working.
Conventional quartz optical fibers for communication, when used with a laser beam having a wavelength of not less than 2 .mu.m, causes large infrared absorption derived from the molecular vibration, resulting in high loss. For this reason, the quartz optical fibers cannot be used as waveguides for transmitting these laser beams. This has led to energetic research and development of new type of optical waveguides for infrared waveband usable in a wide range of applications.
Waveguides, for infrared radiation with a wavelength of not less than 2 .mu.m, which are currently under research and development are classified roughly into solid type, that is, infrared fibers, and hollow waveguides.
Materials for infrared fibers are classified into heavy metal oxide glasses (for example, GeO.sub.2 and GeO.sub.2 --Sb.sub.3 O.sub.3), chalcogenide glasses (for example, As--S and As--Se), and halides. The halides are further classified into halide glasses (for example, ZnCl.sub.2 and CdF.sub.3 --BaF.sub.2 --ZrF.sub.4) and crystalline metal halides (for example, KRS5, AgCl, AgBr, and KCl).
Also for the hollow waveguide, various waveguides different from one another in structure, material and shape have been proposed and produced on an experimental basis. Among others, a metallic hollow waveguide, with a dielectric provided on the inner wall thereof, comprising a high reflective coating provided on the inner wall of a metallic pipe, has been proposed for application to laser machining of high power transmission, and a waveguide having a structure comprising a thin layer of germanium, zinc sulfide or the like formed on the inner wall of a pipe of a metal, such as nickel, has been produced on an experimental basis.
The above hollow waveguide is produced as follows. At the outset, a thin layer of an inorganic material, permeable to infrared radiation, such as germanium or zinc sulfide, is formed by sputtering method on the outer periphery of a base pipe of aluminum or the like which can be etched. Further, a thick nickel layer is formed by electroplating method on the outer periphery of the thin layer of the inorganic material. Finally, the base pipe is removed by chemical etching. Interposition of a thin layer of silver between the thin layer of germanium or zinc sulfide and the thick layer of nickel for ensuring the mechanical strength enables of the preparation of a waveguide with a lower loss.
Up to now, the above construction has realized a transmission loss of 0.05 dB/m and a transmission capacity of 3 kW and has been found to transmit energy required for cutting and welding of metal plates. As compared with the solid type infrared fiber, this hollow waveguide is less likely to cause reflection of the radiation at the time of entry into and emergence from the waveguide and is excellent in cooling effect, rendering the hollow waveguide suitable for transmission of high power infrared radiation.
On the other hand, also in the ultraviolet region, there is a light source, such as an excimer laser, which is important in laser chemistry. Solid type optical fibers, however, cause, in a shorter wavelength, an extreme increase in loss derived from Rayleigh scattering and hence cannot be used as a waveguide. For this reason, research and development of a waveguide for the ultraviolet region has hardly been made in the art.
Solid type optical fibers for use in infrared wavelength region generally have high refractive index, resulting in large reflection loss, and hence are disadvantageous for transmission of high power infrared radiation. In particular, the above conventional glass optical fiber generally has low melting or softening point, and slight loss is likely to cause damage to the end face of the optical fiber. Further, in most cases, the transmission region is in the range of up to 6 or 7 .mu.m, making it difficult to transmit CO.sub.2 laser light. For some crystalline infrared fibers, the transmission region reaches 10.6 .mu.m, a waveband of CO.sub.2 laser. They, however, are likely to cause plastic deformation upon repeated bending and are largely deliquescent, posing a problem of long-term reliability.
The conventional method for making a hollow waveguide provided on its inner wall with germanium, zinc sulfide or the like is complicate and unsuitable for mass production of the hollow waveguide and, further, cannot easily reduce the diameter or increase the length of the hollow waveguide. For the metallic hollow waveguide provided on its inner wall with a dielectric using germanium, zinc sulfide or the like, since the thin layer as the inner layer is formed by sputtering method, the length of the hollow waveguide depends upon the apparatus used for the production of the waveguide and, in the case of actual waveguides, is several meters at the longest. The inner diameter of the waveguide is the outer diameter of the pipe as the base material which is removed by etching in the final step. The pipe as the base material should be completely removed, and, hence, the inner diameter of the waveguide cannot be made very small. At the present time, the smallest possible diameter of the waveguide is about 1 mm. The larger the diameter of the waveguide, the lower the mechanical bendability and the higher the bending loss. Further, a laser beam of more high order modes is propagated, posing a problem of deteriorated focusing properties.
In the ultraviolet waveband, as described above, solid type optical fibers cause, in a shorter wavelength, an extreme increase in loss derived from Rayleigh scattering, and, hence, research and development of a waveguide for the ultraviolet region has hardly been made in the art. Waveguides having a hollow structure, in which Rayleigh scattering is negligible, are considered as a promising waveguide for transmission of ultraviolet light.